Hydrogen sulfide is commonly present in the effluent streams from various processes in chemical plants, refineries and natural gas plants. Typically, the concentration of hydrogen sulfide in such streams ranges from about 20 to about 90 mole percent and occasionally up to 100 mole percent. Other components typically present in such streams include, for example, carbon dioxide, carbon monoxide, carbonyl sulfide, carbon disulfide, water, light hydrocarbons, ammonia, hydrogen cyanide and nitrogen.
It is often necessary to reduce the concentration of hydrogen sulfide in the above effluent streams to meet current environmental regulations which limit the amount of sulfur compounds, such as hydrogen sulfide and sulfur dioxide, which can be emitted to the atmosphere. Typical acceptable levels range from about 10 to 100 ppm of hydrogen sulfide and from about 50 to 1000 ppm of sulfur dioxide.
A common method for reducing the concentration of hydrogen sulfide in process streams is to react hydrogen sulfide with sulfur dioxide to form elemental sulfur and water. This method is generally known in the industry as the Claus process. In the Claus process, hydrogen sulfide is reacted with oxygen, e.g. air, by carefully controlled combustion to form a combustion product having a molar ratio of about two-thirds hydrogen sulfide to one-third sulfur dioxide. During the combustion, some elemental sulfur is formed, which is removed from the combustion product by cooling. Unreacted hydrogen sulfide and sulfur dioxide are passed to one or more catalytic reactors which typically comprise a cobalt-molybdenum catalyst on an alumina support for further conversion to elemental sulfur. A detailed description of the Claus process is provided, for example, by Sander, et al., "Sulfur, Sulfur Dioxide and Sulfuric Acid" The British Sulfur Corporation Ltd. (1984).
One problem associated with the Claus process is that the combustion forms water as a by-product. In addition, when air is used for combustion, the combustion product will comprise nitrogen. Thus, water and nitrogen then comprise a portion of the reactor feed along with feed impurities, e.g. carbon dioxide. These components are inerts in the reaction of hydrogen sulfide and sulfur dioxide to form elemental sulfur and water, and have the effect of reducing the concentration and hence, partial pressure of the hydrogen sulfide and sulfur dioxide in the reactor feed. In fact, when air is used for combustion, 6 moles of nitrogen are introduced in the product per mole of sulfur dioxide formed. Thus, the dilution effect of the nitrogen is significant. As a result, the driving force for the reaction and the reaction rate are lower than a reactor feed without such inerts. Performing the combustion in the presence of pure oxygen or air enriched in oxygen, or increasing the total pressure during the reaction step, can increase the partial pressure of the reactants. However, the costs of providing pure oxygen or air enriched in oxygen can be prohibitively expensive. Furthermore, since the feed streams commonly treated by the Claus process are at essentially ambient pressure, increasing the total pressure requires compression of the feed stream which is often uneconomical. Another problem related to combustion is that the combustion step must be carefully controlled in order to achieve the appropriate concentrations of hydrogen sulfide and sulfur dioxide in the reactor feed.
The Claus process typically operates at reaction efficiencies, i.e. conversion to elemental sulfur, of about 95 to 98 percent. Accordingly, since the conversion is incomplete, the tail-gas often must be treated prior to being emitted to the atmosphere. One method of treating the tail-gas is by simple incineration of the hydrogen sulfide to form sulfur dioxide. However, this method is often unacceptable because the resulting sulfur dioxide concentration usually exceeds the acceptable environmental limits. Another method for reducing the concentration in the tail-gas is to hydrogenate the sulfur dioxide to hydrogen sulfide, recover the hydrogen sulfide, such as by absorption, and recycle the hydrogen sulfide to the feed-end of the Claus process. Another method is to incinerate the hydrogen sulfide in the presence of oxygen to form sulfur dioxide, recover the sulfur dioxide, such as by absorption, and recycle the sulfur dioxide to the feed-end of the Claus process. These options are known in the art and described, for example, by Sander, et al., supra, at pages 56 to 90.
Although the Claus process has been widely used commercially for the removal of hydrogen sulfide from feed streams, improved processes are desired which can reduce the concentrations of water and nitrogen and other impurities in the sulfur conversion reactors and eliminate the need for the carefully controlled combustion step to provide the desired concentrations of feed reactants. In addition, new processes are desired which can be used to increase the throughput of existing Claus process plants and reduce the size of new Claus process plants.